1. Field of the Invention
The invention relates to a method for monitoring a catalytic converter having a catalytic layer, in which a measured variable is ascertained at the catalytic converter and compared with a comparison value, and the comparison value is ascertained in accordance with a model that uses at least one parameter of the catalytic converter.
In German Published, Non-Prosecuted Patent Application DE 197 14 293 A1, corresponding to U.S. patent application Ser. No. 09/056,380, filed Apr. 7, 1998, a method for monitoring the conversion capability of a catalytic converter is described, in which an outlet temperature of the catalytic converter is ascertained as a measured variable, and the measured variable is compared with a comparison value. The comparison value is calculated in accordance with a model that is based on the outlet temperature of a non-catalytically-coated catalytic converter.
The model for calculating the comparison variable is dependent on the catalyst being used, on the motor vehicle operating conditions and on the site in which the catalytic converter is installed in the motor vehicle. That can affect the precision of the method.
A method and an apparatus for assessing the operability of a catalytic converter which are known from German Published, Non-Prosecuted Patent Application DE 42 11 092 A1, utilize a recognition that a conversion onset temperature of the catalytic converter becomes higher as the age of the catalytic converter becomes greater. A first method seeks to investigate when a catalytic converter begins to convert and to assess it as operational if the ascertained temperature is below a comparison temperature. A second method is disclosed in which the catalytic converter is operated under precisely predetermined operating conditions, and an investigation is directed as to how long it takes until the catalytic converter begins to convert. If the measured time period is shorter than a comparison time period, the catalytic converter is operational. A third method seeks to estimate the temperature of the catalytic converter using a model. As soon as the estimated temperature has attained a comparison temperature, an investigation is directed as to whether or not the catalytic converter has exceeded that temperature. If so, it is assessed as operational.